1. Field of the Invention
The invention concerns a method for generation of digital x-ray image exposures and an x-ray image acquisition device for implementing such a method.
2. Description of the Prior Art
X-ray image acquisitions of the human breast normally ensue with mammography apparatuses. Medical examinations of the soft tissue of the human breast with x-ray radiation that serve for early detection of breast cancer can be conducted with a mammography apparatus. The breast to be examined is clamped between a subject table and a compression plate that can be displaced against the subject table. An x-ray image acquisition subsequently ensues with an exposure unit fashioned as an x-ray radiator. For this purpose, an x-ray detector is typically integrated into the subject table. Radiation known as soft x-ray radiation, in the range below 50 kV, is used in the acquisition.
Modern mammography apparatuses (for example the “MAMMOMAT Novation” that is commercially available from Siemens Healthcare) have a base body fashioned as a support and a curved apparatus arm projecting from the support, at the free end of which is arranged a radiation source. The apparatus arm is formed as a plate structure and is connected in a torque-resistant manner with a horizontal rotation axis of the mammography apparatus, so that the radiation source can be pivoted by 360° around an isocenter. A subject table that can be pivoted by 360° around the isocenter is supported on the apparatus arm by a revolving joint.
The mammography apparatus is typically used for examinations known as screening examinations in which the exposure unit is located in a 0° position, with the exposure unit and the subject table arranged opposite one another in the longitudinal direction. Furthermore, the mammography apparatus is also fashioned for a stereo image examination in which the breast is exposed from two different angles. Here the exposure unit is pivoted by +/−10° or by +/−15° from the rest position around the horizontal axis given a stationary subject table. Furthermore, often examinations known as tomosynthesis examinations are also possible with the mammography apparatus, in which the exposure unit travels continuously over a relatively large angle range, for example in an angle range from +/−25° around the horizontal axis given a stationary subject table. Furthermore, an MLO presentation (medio-lateral-oblique) is possible. In this examination, the exposure unit is normally found in a 45° position, and the subject table follows the exposure unit so that subject table and exposure unit are always aligned in the same position and at the same distance relative to one another. The mammography apparatus therefore allows the acquisition of the breast to be examined in standard presentations such as what is known as the cranio-caudal (CC) or medio-lateral-oblique (MLO) presentation, for example.
The “MAMMOMAT Novation” enables a stereotactic biopsy of the human breast in cooperation with the “Opdima” digital biopsy and target acquisition system by the applicant. In stereotactic biopsy a tissue sample of the human breast is extracted and subsequently histologically examined.
For a biopsy, the breast is compressed in the mammography apparatus with a compression plate specially fashioned for the biopsy and is fixed in this position. A first x-ray image acquisition (known as the overview acquisition) subsequently ensues in order to determine whether the tumor or another tissue part to be biopsied lies within a recess of the compression plate.
All following acquisitions ensue in pairs from two different directions, as described above. They are therefore designated as stereo acquisitions. From the first stereo acquisition, the point in the breast that is to be biopsied is marked, and the required penetration position, penetration direction and penetration depth of a biopsy needle are calculated from this. The biopsy needle is subsequently inserted into the breast at the calculated point with the aid of a biopsy device, and additional stereo exposures are shot to monitor the position of the needle tip.
The biopsy needle and biopsy needle mount are therefore imaged in these additional stereo exposures. Both overlap a large portion of the tissue surface of the breast in the x-ray image exposures. Therefore, automatic contrast and brightness adaptations which are based on these exposures deliver no constant image quality. Moreover, no automatic exposure correction (AEC) is possible in these exposures.
In most cases, the x-ray image acquisition is manually windowed by an operator in order to achieve a satisfactory image quality. The duration of an examination is thereby unnecessarily extended, which is uncomfortable for the patient and increases the costs of a biopsy.
In “Opdima”, metal artifacts are segmented via a histogram analysis. This segmented region can then be excluded for a contrast and brightness adaptation.